Narrow-band optical filters are commonly used in many fields, such as remote sensing, optical astronomy, spectroscopy, and optical communication. These filters isolate a narrow spectral band from a wide spectral band radiation source and unwanted background light sources. Isolation of a narrow spectral band may aid in detecting a particular characteristic from a subject to be investigated or in multiplexing or demultiplexing channels in optical communication.
Existing narrow-band filters typically use a dispersing system and an array of photodetectors, such as a charge coupled device (CCD), to separate a beam into several spectral bands of the size Δλ. Each spectral band is detected by a separate photodetector. The array of photodetectors, therefore, detects a desired spectral region, ΔΛ.
Generally, if a greater resolution of a spectral band is needed, the size Δλ of each spectral band is decreased, causing the total spectral region ΔΛ to also decrease. Thus, only bands close to each other on the spectrum are detected by the array at high resolution. Applications requiring high resolution detection of widely separated spectral bands can therefore be difficult. Another problem with these conventional narrow-band filters is that high resolution can require a small solid angle of detection, thus limiting sensitivity.
Other devices for narrow-band detection of selected spectral lines include Fabri-Perot etalons or interference filters. Although these devices may detect narrow spectral bands, they have a high angular selectivity, which decreases their sensitivity. Further, they typically use special glass filters for mirror substrates that cause the filters to operate at only one wavelength. Thus, only a single spectral line is detected.
Another type of spectral filter is a Bragg grating (BG). BGs can also have strong angular selectivity, causing insensitivity. This problem was addressed by G. A. Rakuljic, V. Leyva, “Volume Holographic Narrow-Band Optical Filter,” Opt. Lett., v. 18, pp. 459-461, 1993 (“Rakuljic”). Use of spectral filters, such as Rakuljic, however, teaches introduction of a beam splitter between the incident light and a holographic filter, reducing the post-filter signal power to a quarter of what it would be without the beam splitter. Further, a portion of the beam reflected off of the holographic filter may return to the source of the incident beam, which can be strongly undesirable.
Accordingly, a need exists for a narrow band filter that can detect a set of pre-selected wavelengths at high resolution through a single receiving channel. A further need exists for a filter that can detect pre-selected wavelengths that are spectrally distant from each other. Needs also exist for a filter with a wide angular field of view and high sensitivity at a variety of angles of incident light. These and other needs are addressed by one or more embodiments of the present invention.